Pre-Location vs Pinpointing: The Two Essential Steps of Cable Fault Location
Every professional cable fault location job follows the same two-step process:
- Pre-location — narrow the fault down to a specific distance
- Pinpointing — find the exact spot to dig
New technicians often wonder why both steps are needed. Why can't you just walk the cable with one tool and find the fault? The answer is efficiency: pre-location covers kilometers in minutes; pinpointing covers meters in minutes. Used together, they turn a multi-day repair into a same-day fix.
What Pre-Location Does
Goal: Determine the distance to the fault along the cable.
Primary tool: Time Domain Reflectometer (TDR)
The TDR sends a pulse down the cable and measures the distance to the fault reflection. The result is a number: "the fault is at meter 780."
Also used for pre-location:
- High-voltage bridge methods (Murray loop, etc.) for specialized fault types
- Cable length confirmation
Time required: minutes per cable
Result: a search distance, accurate to ±0.5 m along the cable — but not a physical ground location.
What Pinpointing Does
Goal: Find the exact location on the ground surface where excavation should begin.
Primary tool: Acoustic-magnetic pinpointer
The pinpointer works with an impulse generator (thumper) that stresses the fault. The fault discharges with an audible "thump," and the pinpointer detects:
- The acoustic signal — strongest directly above the fault
- The magnetic signal — travels instantly to the fault, giving a time reference
Where the two align is the dig point.
Also used for pinpointing:
- Earth gradient methods for sheath faults
- Manual probing in specific cases
Time required: 30–60 minutes for a typical fault
Result: an exact surface location, accurate to under half a meter
Why Both Steps Are Required
Consider a 2 km feeder with a fault.
- Without pre-location: you would walk the full 2 km with a pinpointer, listening for thumps. On a deep or quiet fault, this could take all day — or you might miss it entirely.
- Without pinpointing: you would excavate at "meter 780" along the surface — but cables rarely run in a straight line. The actual dig point could be 5, 10, or 50 meters from where the tape measure says 780.
The two steps together:
Integrated Systems Make It Simple
Rather than juggling separate instruments, complete fault location systems integrate TDR, impulse generator, and pinpointer into one package:
- Portable systems — transportable kits for contractors (380V–35kV, or up to 500kV for transmission work)
- Trolley-mounted systems — all-in-one mobile rigs for utilities
- Vehicle-mounted systems — complete rapid-response units for large networks
All three steps stay in sync, setup is faster, and training is simpler — one system instead of three separate tools.
Browse the full range of portable and mounted test systems at TriEdge Technologies.
FAQ
Can I skip pre-location and just use a pinpointer? On cables under a few hundred meters, sometimes. On longer feeders, walking the full length with a pinpointer is impractically slow. Pre-location first is the professional standard.
Why does my TDR show a distance but I can't find the fault on the ground? Because the cable doesn't follow the surface distance exactly — it curves, crosses roads, and passes under structures. The TDR distance is along the cable, not the surface. Pinpointing resolves this.
How accurate is pre-location? Professional TDRs achieve ±0.5 m along the cable. This is enough to know which stretch to search, but not enough to dig without pinpointing.
Do I need a separate impulse generator for pinpointing? Yes — the pinpointer detects the fault's discharge, which requires the fault to be stressed by an impulse generator. Complete systems include both.